It has long been known to manufacture and distribute pressure-sensitive adhesive stock for display products such as labels and signs by providing a layer of face material for the label or sign backed by a layer of pressure-sensitive adhesive which in turn is covered by a release liner. The liner protects the adhesive during shipment and storage. With specific reference to labels, the liner also allows for efficient handling and dispensing of individual labels which have been die-cut from the layer of face material while leaving the liner uncut.
Many label and sign applications require that the face material be a polymeric film material which can provide properties lacking in paper, such as weatherability (for outdoor signs), strength, water resistance, abrasion resistance, gloss and other properties. Because material costs in the manufacture of such film facestocks are relatively high, the desirability of reducing material costs without sacrifice of quality has long been apparent, but little or nothing has been accomplished toward this end.
Because the cost of paper generally compares favorably with the cost of film materials, and because paper liners also have other highly desirable characteristics, the liners used with film facestocks have generally comprised paper web stock coated with a very thin layer of silicone-based release agent. The paper web's outer or "back" face has the roughness required to track well on the smooth steel rolls used in high speed manufacturing. The inner release-coated face of the paper web is uneven enough to slightly roughen the surface of the adhesive protected by the liner, thus preventing subsequent air entrapment and bubble formation between label and container in labeling applications.
However, paper readily absorbs and desorbs moisture, leading to curling and distortion of film facestock with which a paper liner is used. This is particularly a problem with sheet facestock used, say, for signs and decals. Moisture absorption and curling have been reduced to a degree by coating the outside face of the paper liner with a thin moisture barrier layer of film material, but edge absorption or gradual moisture transmission through the moisture barrier itself have largely thwarted efforts to eliminate the problem of curling of the liner and consequent distortion of the film facestock.
Another disadvantage of paper is its relative mechanical weakness. This is particularly a drawback in high speed packaging of high volume consumer products where labeling machinery must dispense rolls of liner-carried labels at high speed. A break in the paper liner forces shutdown of the entire packaging line until the labeling operation is properly reset. As line speeds have continued to increase in recent years, the severity of this problem has led some mass packagers to specify that labels are to be carried on polyester film liner. The great strength of the polyester film eliminates the liner breakage problem, but at a price which reflects very much higher material costs than those associated with paper.